A thin film transistor (TFT) has been used widely as a switching device for a display of a liquid crystal display apparatus or the like. A representative TFT comprises a gate electrode, an insulating layer and a semiconductor layer sequentially on a substrate, and a source electrode and a drain electrode formed on the semiconductor layer with a predetermined interval. The organic semiconductor layer constitutes a channel region, and a TFT performs an on-off operation by adjusting current flowing between the source electrode and the drain electrode by a voltage applied to the gate electrode.
Conventionally, a TFT was fabricated by using amorphous or polycrystalline silicon. However, a CVD apparatus which is used for fabricating a TFT using silicon is very expensive. Therefore, there was a problem that an increase in size of a display apparatus or the like using a TFT resulted in a significant increase in production cost. Further, since forming an amorphous or polycrystalline silicon into a film is conducted at a significantly high temperature, kind of materials which can be used as a substrate is limited, which makes the use of a light-weight resin substrate or the like impossible.
In order to solve such problem, a TFT which uses an organic substance instead of amorphous or polycrystalline silicon (hereinafter often abbreviated as an organic TFT) has been proposed. As the film-formation method when a TFT is fabricated by using an organic substance, a vacuum vapor deposition method, a coating method or the like are known. According to these film-formation methods, a device can be increased in size while suppressing an increase in production cost, and the process temperature necessary for film formation can be suppressed to a relatively low temperature. Therefore, an organic TFT has advantages that less restriction is imposed on the selection of materials used in a substrate. As a result, practical application of an organic TFT is expected, and intensive studies have been made thereon.
As for the organic semiconductor used in an organic TFT, as the material for a p-type FET (field effect transistor), a polymer such as a conjugated polymer and thiophene, a metal phthalocyanine compound, a fused aromatic hydrocarbon such as pentacene or the like are used singly or in the form of a mixture with other compounds. As the material for an n-type FET, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA), 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalene tetracarboxylic diimide (NTCDI) or fluorinated phthalocyanine are known, for example.
On the other hand, as the device which similarly utilizes electric conductance, an organic electroluminescence (EL) device is known. In an organic EL device, generally, electric charge is flown forcibly by applying a strong electric field of 105 V/cm or more in the film thickness direction of an ultrathin film of 100 nm or less. In the case of an organic TFT, electric charge is required to be flown at a high speed in an electric field of 105 V/cm or less for a distance of several μm or more. Therefore, an organic substance itself used in an organic TFT is required to be further conductive. However, the above-mentioned compounds used in conventional organic TFTs have a small field effect mobility and a slow response speed, and hence, the high-speed responsiveness thereof as a transistor is not satisfactory. In addition, it has a small on-off ratio.
The “on-off ratio” as referred to herein means a value obtained by dividing current flowing between a source and a drain when applying a gate voltage (ON state) by current flowing between a source and a drain when applying no gate voltage (OFF state). The ON current normally means a current value (saturation current) at the time when current flowing between a source and a drain is saturated when the gate voltage is increased.
In order to obtain a TFT having a high electric field effect mobility and a high response speed, a compound having an arylethynylene group is disclosed in Patent Document 1, for example. In this document, in the fabrication of a device, a high mobility is obtained by forming a monolayer on the substrate, and conducting vapor deposition while heating the substrate. However, this method had a problem that the processing is complicated.
As the representative example of the material for this organic TFT, pentacene can be given. In Patent Documents 1 and 2, an organic TFT in which pentacene is used in an organic semiconductor layer is fabricated. Since pentacene had a defect that its stability in the atmosphere is low, although the mobility of a device was significantly high immediately after the device fabrication, it fell with the passage of time. Organic TFTs using picene which is a fused aromatic ring are reported in Non-Patent Document 1. This document states that, since picene has a lower ionization potential than pentacene, it has superior oxidation stability in the air. However, when picene was used, although a device showed a mobility of 1.0 cm2/Vs, the use of picene had such defects that a high driving voltage of −67 V, is required to be applied.